1. Field of the Invention
The present invention relates to a photoelectric conversion device and an image pickup device.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2003-229557 describes an image sensor in which pixels are two-dimensionally arrayed. In the block diagram of FIG. 10, a plurality of pixels share an amplifier/noise processing circuit (signal processing portion) 14. FIG. 11 shows the concrete circuit arrangement of the signal processing portion described in Japanese Patent Laid-Open No. 2003-229557.
Generally, blocks for reading out a signal from a signal processing portion to a common signal line while a horizontal scanning circuit selects (scans) columns are sequentially arranged in the order of the signal processing portion, the common signal line, and the horizontal scanning circuit. With this arrangement, signals can be read out from pixels via a short path without the intervention of the horizontal scanning circuit.
In the arrangement described in Japanese Patent Laid-Open No. 2003-229557, the signal processing portion, e.g., amplifiers 14d and 14e shown in FIG. 11 needs to receive power. According to examinations of the present inventors, the following problems are found in the arrangement of power lines for supplying power.
First, as the resistance of the power line for the signal processing portion increases, the output from an OB (Optical Black=black reference pixel) fluctuates in accordance with horizontal shading and the optical signal of an effective pixel, thus posing a problem. Assume that a current of 10 μA per column flows in order to operate the signal processing portion. In this case, a solid-state image sensor with a resolution of 10 million pixels requires a signal processing circuit with about 4,000 columns. Although the signal processing circuit is horizontally divided, one of the signal processing circuits has 2,000 columns, and a current of 20 mA flows in total. Because of a voltage drop caused by the current, the voltage of the power supply shifts by several hundred mV in accordance with the position in the horizontal direction. As a result, horizontal shading is generated.
Second, a problem occurs when turning off an unused signal processing circuit in order to suppress power consumption. In this case, when the signal processing circuit switches from an OFF state to an ON state, it requires a period of time proportional to the product of the resistance of the power line and the sum of the substrate capacitance and power capacitance of the signal processing circuit, until the circuit stabilizes.
Third, the charge/discharge of capacitance and the current flowing through the signal processing circuit fluctuate in accordance with the output from each row. As described above, it takes a period of time until the fluctuation stabilizes, and high-speed reading cannot be performed, thus posing a problem. More specifically, in accordance with the output from the signal processing circuit, a current consumed by the signal processing circuit in a DC manner fluctuates by, e.g., about 1% with channel-length modulation of a MOS transistor used in the signal processing circuit. When a current flowing through the signal processing circuit is 20 mA, 1% of this current is 0.2 mA. When this current is voltage-converted by a resistor of 10Ω, a voltage of 2 mV is obtained. That is, the black reference fluctuates as much as 2 mV in accordance with the output, thus posing a large problem. Because of these problems, an output image becomes similar to an image degraded in quality due to smear in a CCD. Note that, in principle, smear occurs in the longitudinal direction in the CCD, and in the lateral direction in an amplification type solid-state image sensor.
In order to simply reduce the resistance of the power line, a wiring layer for the power line may be newly formed. More specifically, the horizontal scanning circuit is wired on the first and second metal layers, and the third metal layer is only used for wiring the power line. In this case, the third metal layer can be formed above the horizontal scanning circuit. However, the parasitic capacitance of a common signal line becomes about twice if the third metal layer covers the common signal line. As a result, the performance largely degrades, e.g., the readout speed decreases, and the readout gain decreases to ½.